This invention relates to injection molded foamed structures, and more particularly to injection molded fan blades for use in the fan assembly of a helicopter equipped with a mechanical anti-torque directional control system in which no tail rotor is employed.
A concept has been developed for countering main rotor torque in helicopter propulsion systems by replacing the traditional tail rotor with a constant pressure circulation control tail boom. Sideward thrust is controlled by a thruster valve located at the aft end of the tail boom. A variable pitch fan mounted at the forward end of the tail boom provides the required airflow to the thruster. One particular version of such a mechanical antitorque directional control system, denoted by the trademark NOTAR, has been developed by McDonnell Douglas Helicopter Co. of Mesa, Arizona, and is more completely described in U.S. Pat. Nos. 4,200,252 and 4,948,068, herein incorporated by reference.
The variable pitch fan for the NOTAR.RTM. control system referred to above incorporates a plurality of fan blades, preferably thirteen, mounted about a fan hub having means for effecting pitch control for the blades. The blade as originally designed consists of a foam core, a fiberglass epoxy skin, and an abrasion strip bonded to the leading edge. Total cycle time for building one composite blade is more than eight hours, which includes two hand lay-ups, two oven cures, two trims, one painting cycle, and a secondary bonding of the abrasion strip to the blade leading edge. Because of the labor intensive manufacturing process, fabrication costs approximate $500 per blade, with a production scrap rate ranging close to 50 percent. In addition, the composite fan blade has a brittle foam core which is susceptible to impact damage during handling and in operation.
Prior art injection molding processes, while capable of producing fan blades much more inexpensively than is possible using the above described composite process, would tend to produce blades having inadequate strength characteristics and/or excess weight.
What is needed, therefore, is a fan blade, and a process for making same, which will be relatively inexpensive while having adequate strength, weight, and durability characteristics to ensure dependable operation in a crucial, demanding environment.